ZINC ORES OF THE EDWARDS DISTRICT 27 



manifest, the magnitude of this difference is by no means obvious; 

 but there seems to be good reason for believing that the light blende 

 marks a late stage of the main period of ore formation rather than 

 a widely separated recurrence of ore-forming conditions. 



This is indicated by the intimate association of the light blende 

 with the younger serpentine. The two minerals are so closely 

 mingled in veinlets cutting the older mineral as to suggest very 

 strongly their essentially contemporaneous deposition, the blende 

 being, if anything, a little older than the serpentine. As previously 

 stated, cracks in the older blende are usually filled with calcite or 

 the younger serpentine and there was evidently an abundant supply 

 of these minerals, amply sufficient to fill all cracks. If this had 

 happened, it is hardly possible that the younger blende, always in 

 small amount, could have found its way into these cracks and 

 replaced serpentine and calcite. On the contrary, the conclusion 

 seems unavoidable that the younger blende was deposited in the 

 cracks slightly before, or simultaneously with, the other minerals. 



Occasionally the light blende, in contrast with the dark, replaces 

 calcite with a marked development of the rhombohedral form 

 (plate 12, figure i). This occurs when cleavage cracks have formed 

 in the calcite as a result of the same disturbances that crushed the 

 older blende, these cleavages controlling the flow of solutions and 

 deposition of the blende. The phenomenon is a very minor one, 

 marking a late stage in the period of ore formation. Very rarely, 

 the light blende may be found replacing barite (plate 12, figure 2). 



In only one case, so far as seen, does the light blende become the 

 dominant type of ore, and here its history seems to be quite unlike 

 that sketched above. 



The ore in question occurs near Sylvia lake and was opened 

 up many years ago as a source of iron. It has turned out to be 

 the oxidized portion of a zinc deposit and, in depth, the hema- 

 tite is giving place to pyrite, while blende is appearing in some 

 quantity. As might be expected, calcite has been removed by 

 the free circul?tion of ground waters to which the ore has been 

 subjected, and samples from the bottom of the pit are an aggre- 

 gate of vein quartz, hematite, pyrite and blende. This is very 

 different from the ordinary zinc ores but the difference is doubtless 

 due largely, if not wholly, to secondary causes. The deposit has 

 been subjected to the attack of circulating superficial waters and 

 largely made over by them. Much material has been removed in 

 solution and what remains has been dissolved and reprecipitated. 



